Conveyance system and method of controlling the same

ABSTRACT

A conveyance system which performs conveying action with operating a part of or all of a plurality of carrier vehicles, including a loop track, a plurality of carrier vehicles which run on the track, a station where each carrier vehicle performs loading and unloading of a cargo, and a standby station for storing the carrier vehicles at their standby state. The number of the carrier vehicles to be used is determined from a conveyance command number, and can be automatically determined corresponding to the quantity of load in the conveyance system. Accordingly, even if the load is not uniform but significantly varied, the conveying action can be conducted readily and efficiently.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a conveyance system forconveying cargoes in a plant or warehouse and particularly to aconveyance system in which carrier vehicles run along a track(s)provided on the floor or the ceiling for conveying cargoes.

[0002] A conventional conveyance system in which a plurality of carriervehicles run on a track(s) for conveying cargoes is disclosed inJapanese Unexamined Patent Publication No. 2000-353015 (referred to asPrior Art 1 hereinafter). The conventional conveyance system of PriorArt 1 will be explained referring to FIG. 13.

[0003] The carrier vehicles 106 (106 a, 106 b, 106 c, . . . ) run on atrack 102 provided on the floor or the ceiling to convey cargoes to andfrom warehouses 101 (101 a, 101 b, 101 c, . . . ). There are stations103 (103 a, 103 b, 103 c, . . . ), each for one of the warehouses, wherethe carrier vehicles 106 stop to load and unload their cargoes. Thetrack 102 has switches to form double-track sections where the stations103 are sited. This allows the carrier vehicles to run continuously butnot to be interrupted by any other carrier vehicle which stops to loador unload its cargo.

[0004] In the conveyance system of Prior Art 1, the carrier vehiclehaving unloaded its cargo stands by at the station before it receives acommand. For example, as shown in FIG. 13, the carrier vehicle 106 b hasunloaded its cargo for storage in the warehouse 101 a 1and stands by atthe station 103 a before it receives another command.

[0005] If the other carrier vehicle 106 a arrives for storage in orpicking up from the warehouse 101 a, the vehicle 106 b has to be movedfrom the station 103 a. The conveyance system of Prior Art 1 has acommunication point 107 a provided on the track 102 for detecting thepassage of the carrier vehicle 106 a and when finding that thedestination of the carrier vehicle 106 a is the station 103 a,commanding the carrier vehicle 106 b to move from the station 103 a toanother free station (e.g. 103 b).

[0006] Another conventional conveyance system is disclosed in JapanesePatent Unexamined Publication No. 180610/1984 (referred to as Prior Art2 hereinafter) where a plurality of carrier vehicles run on a track(s)to convey cargoes. The conventional conveyance system of Prior Art 2will be explained referring to FIG. 14.

[0007] In the conveyance system of Prior Art 2, a carrier vehicle 106runs on a track 102 to convey cargoes between stations 103.

[0008] It is now assumed that the carrier vehicle 106 arrives at thestation 103 which is busy or out of order thus allowing no unloading.Then, the carrier vehicle passes the station and loops the track beforecoming back again. If the station fails to eliminate its busy orout-of-order condition by then, it forces the carrier vehicle to loopthe track once more without unloading its cargo.

[0009] When such a condition remains for a considerable length of time,the carrier vehicle has to run on the track continuously and decline itsoperating efficiency. For compensation, the conventional conveyancesystem of Prior Art 2 has a transmitter 108 provided on the track 102for supplying each carrier vehicle with a signal upon its passing by tocount a number of passage. When the number of passage exceeds a givennumber, the carrier vehicle 106 is commanded to move to and stay at anassigned station 105.

[0010] In case the carrier vehicles are too many on the track ascompared with the number of cargoes to be conveyed, they mightundesirably meet each other at the switches or stations thus increasingthe standby duration and interrupting the smoothness of conveyingactions. Also, the carrier vehicles run without cargoes at morefrequency and its operating efficiency will be declined. If the carriervehicles are few on the track as compared with the number of cargoes tobe conveyed, the waiting for the carrier vehicles will be extended henceinterrupting the smoothness of conveying actions.

[0011] It is therefore desired to determine the number of the carriervehicles depending on the number of cargoes to be conveyed or the totalload to be handled by the conveyance system. However, the number ofcargoes is not constant but usually varied time to time. Theconventional conveyance system of Prior Art 1 or 2 has a predeterminednumber of the carrier vehicles provided for use regardless of the totalload of cargo to be conveyed and its smooth, efficient conveying actionmight be interrupted by an abrupt change in the load.

SUMMARY OF THE INVENTION

[0012] It is hence an object of the present invention to provide aconveyance system which can automatically control the number of carriervehicles corresponding to a change in the load to be handled.

[0013] For achievement of the object of the present invention, theconveyance system which performs conveying action with operating a partof or all of a plurality of carrier vehicles includes a loop track, aplurality of carrier vehicles which run on the track, a station whereeach carrier vehicle performs loading and unloading of a cargo, and astandby station for storing the carrier vehicles at their standby state.The number of the carrier vehicles to be used is determined from aconveyance command number.

[0014] A method of controlling conveyance system according to thepresent invention includes the steps of: counting the number ofconveying actions either in operation and to be carried out; calculatingan optimum number of the carrier vehicles corresponding to the number ofconveying actions; counting the number of the carrier vehicles at theirin-action state; and when the number of the carrier vehicles at theirin-action state is smaller than the optimum number, using some carriervehicles at their standby state.

[0015] Another method also includes the steps of: counting the number ofconveying actions either in operation and to be carried out; calculatingan optimum number of the carrier vehicles corresponding to the number ofconveying actions; counting the number of the carrier vehicles at theirin-action state; and when the number of the carrier vehicles at theirin-action state is greater than the optimum number, shifting some of thecarrier vehicles from the in-action state to the standby state.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a schematic layout diagram of a conveyance systemaccording to the present invention;

[0017]FIG. 2 is a diagram showing an arrangement of check points in theconveyance system with the layout shown in FIG. 1;

[0018]FIG. 3 is a diagram explaining the arrangement of a conveyancecontroller in the conveyance system of the present invention;

[0019]FIG. 4 illustrates an example of a table of conveyance commands;

[0020]FIG. 5 illustrates an example of a table for determining thenumber of carrier vehicles;

[0021]FIG. 6 illustrates an example of a table of the status of thecarrier vehicles;

[0022]FIG. 7 illustrates an example of a table of the distance betweenthe check points;

[0023]FIG. 8 illustrate an example of a table of determining thepriority of the carrier vehicles;

[0024]FIG. 9 is a flowchart explaining a controlling action for thenumber of the carrier vehicles according to the present invention;

[0025]FIG. 10 is a flowchart explaining a maintenance action for thecarrier vehicle according to the present invention;

[0026]FIG. 11 is a schematic layout diagram of a conveyance systemaccording to the present invention;

[0027]FIG. 12 is a schematic layout diagram of a conveyance systemaccording to the present invention;

[0028]FIG. 13 is a diagram showing a conventional conveyance system ofPrior Art 1; and

[0029]FIG. 14 is a diagram showing a conventional conveyance system ofPrior Art 2.

DETAILED DESCRIPTION

[0030] Some embodiments of the present invention will now be describedreferring to the accompanying drawings.

Embodiment 1

[0031]FIG. 1 illustrates a schematic layout of the conveyance system ofthis embodiment.

[0032] There are a plurality of warehouses (storage shelves) 1 (1 a, 1b, . . . ) and a track 2 extending along the warehouses 1 in a plant.The track 2 might be designed for clockwise traveling. Carrier vehicles6 run on the track 2 for conveying cargoes from one warehouse toanother. A plurality of stations 3 (3 a, 3 b, . . . ) are provided eachfor one of the warehouses. The carrier vehicles 6 stop at the stations 3for loading or unloading their cargoes.

[0033] In general, the track 2 has switches to form double-tracksections where the stations 3 are sited. This allows the carriervehicles not to be disturbed by any other carrier vehicle which is busyto load or unload its cargo. Each double-track section is accompaniedwith a standby station 4 (4 a, 4 b, . . . ). In case the carriervehicles are too many as compared with the total load to be handled bythe conveyance system, their extras are held at the standby stations 4.Also, a maintenance station 5 is provided on the track 2 for carryingout a maintenance action for the carrier vehicles 6.

[0034] The standby stations 4 a to 4 f shown in FIG. 1 share some routeswith the stations 3 a to 3 f respectively. As a result, the carriervehicles can be guided into the standby stations 4 a to 4 f via theircorresponding stations 3 a to 3 f. Any of the carrier vehicles whichneeds no stopover in the standby stations 4 a to 4 f can be returnedback directly to the track 2 from the stations 3 a to 3 f.

[0035] Alternatively, the standby station 4 might be providedseparately, such as a standby station 4 g in FIG. 1, but not shared withthe station 3. Although the maintenance station 5 is located on a branchof the track 2 as shown in FIG. 1, it might be provided on thedouble-track section like the station 3 or the standby station 4.

[0036] Moreover, check points (CP) are provided on the track forexamining the position of the carrier vehicles and temporarily stoppingthe carrier vehicles. FIG. 2 illustrates an arrangement of the checkpoints in the conveyance system shown in FIG. 1. In FIG. 2, twelve ofthe check points (CP1 to CP12) are arranged on the track 2. Each of thecheck points has a carrier passage detecting means which detects thepassage or arrival of the carrier vehicle and transmits its detectionsignal to a conveyance controller described below. The passage detectingmeans might be selected from micro-switches, infrared ray sensors, ormagnetic sensors. It is suitable to have a transmitter provided on thecarrier vehicle for transmitting a very small level of electromagneticwaves which is then received by the check points.

[0037] The conveyance controller 12 in the conveyance system of thisembodiment is designed for controlling the running action of the carriervehicles 6 and the loading/unloading action at each station 3. Theconveyance controller 12 is controlled by a control device such as anupper-stage computer 10, which releases commands of the conveyingactions and receives data about the actions. A schematic construction ofthe conveyance controller 12 is shown in FIG. 3. The conveyancecontroller 12 includes a central processing unit 14 and a communicationcontrol interface 16 for communication with the conveyance system aswell as a group of tables 20 to 28 in which various data including theconstruction of the conveyance system, the current condition ofconveying actions, and the status of the carrier vehicles are memorized.

[0038] The tables will now be described in more detail referring toFIGS. 4 to 8.

[0039] As shown in FIG. 4, the conveyance command table 20 stores eachcombination of the dispatch station and the destination station forconveying a cargo, the number of the carrier vehicle to be used, and thecurrent status of the conveying action. When a command for conveying acargo is issued from the upper-stage computer 10 (e.g. for managing thefull production lines of a plant), its start location (the dispatchstation) and goal location (the destination station) are stored in theconveyance command table 20. Then, the conveyance controller 12determines the carrier vehicle to be used and starts the conveyingaction. When the carrier vehicle has arrived and finished its unloadingaction at the destination station, the command is deleted from theconveyance command table 20. Alternatively, a flag indicating that theconveying action has been finished might be used. Accordingly, all theactions including the current conveying action and the planned actionsto be conducted can be indexed by referring to the conveyance commandtable 20.

[0040] Referring to FIG. 5, the carrier vehicle number determining table22 stores the optimum number N₁ of the carrier vehicles determinedcorresponding to a conveyance command number n. For example, as shown inFIG. 5, when the conveyance command number ranges from 1 to 5, seven ofthe carrier vehicles shall be used. Alternatively, when the conveyancecommand number ranges from 6 to 10, twelve of the carrier vehicles shallbe used. The conveyance command number n means the total amount of loadto be conveyed at the present and the future, and can be calculated fromthe conveyance command table 20.

[0041] Referring to FIG. 6, the carrier vehicle status table 24 storesthe total mileage, the current status, and the location of each carriervehicle. The current status is selected from three different states:“under maintenance” on the maintenance station 5, “under standby” on thestandby station 4, and “in-action”. Also, the “in-action” state isselected from three different types: “occupied” which is loaded with acargo, “reserved” which is on the way to a dispatch station forreceiving a cargo, and “free” which waits for another command whilerunning on the track.

[0042] Referring to FIG. 7, the check point (CP) distance table 26stores the distance between two check points. When the carrier vehiclehas passed or arrived at one check point, the identification number ofthe carrier vehicle is transmitted together with the number of the checkpoint to the conveyance controller 12. The conveyance controller 12updates the location of the carrier vehicle in the carrier vehiclestatus table 24 with the number of the check point received, calculatesthe distance from the current check point to the preceding check pointfrom the CP distance table 26, and adds the distance to the mileage ofthe carrier vehicle in the carrier vehicle status table 24. By repeatingthis procedure, the location and the mileage of each carrier vehicle canbe managed.

[0043]FIG. 8 shows the carrier vehicle priority type table 28. Thecarrier vehicle priority type table 28 stores priority type data forselecting one of the carrier vehicles for either the standby and theaction. For example, as shown in FIG. 8(b), when the priority type is“1”, the minimum mileage vehicle is selected. When the priority type is“2”, the maximum mileage vehicle is selected. When the priority type is“3”, the carrier vehicle located nearest to the destination station isselected. FIG. 8(a) illustrates that the priority type “1” is based onthe standby state while the priority type “2” is based on the in-actionstate. Accordingly, when the standby state is concerned, the priority isgiven to the minimum mileage carrier vehicle. When the in-action stateis concerned, the priority is given to the maximum mileage carriervehicle.

[0044] According to the data in the tables 20 to 28, the centralprocessing unit 14 generates commands to the carrier vehicles 6 and thestations 3. The commands are transmitted via the communication controlinterface 16 to the carrier vehicles 6 and the stations 3 fordetermining their actions. Upon completing the commanded actions such asmoving to the destination, or loading/unloading cargoes, the carriervehicles 6 and the stations 3, 4, and 5 give signals of arrival orcompletion of loading/unloading to the conveyance controller 12.

[0045] A procedure of controlling the number of the carrier vehiclesaccording to the present invention will now be described referring tothe flowchart of FIG. 9.

[0046] The conveyance controller 12, when receiving the signal ofcompletion of unloading from the station 3, acknowledges the end of oneconveyance action. The conveyance controller 12 then deletes the commandfor the action from the conveyance command table 20 (or records the endof the conveyance action) and calculates the conveyance command number nat the current time.

[0047] This is followed by the conveyance controller 12 calculating theoptimum number N₁ of the carrier vehicles to be used from the conveyancecommand number n determined with the carrier vehicle number determiningtable 22, and then the number N₂ of the carrier vehicles in their“in-action” state.

[0048] When the number N₂ is greater than the optimum number N₁ (N₂>N₁),the carrier vehicles in action are too many for handling the load in theconveyance system. Then, some of the carrier vehicles have to be drivenback to the standby stations for standby actions. The conveyancecontroller 12 accesses the carrier vehicle priority table 28 to examinethe standby priority type flag F_(w). When the standby priority typeflag F_(w) is 1, the conveyance controller 12 selects from the carriervehicle status table 24 one of the carrier vehicles which is free andhas the minimum mileage and commands it to move to the standby station.When the standby priority type flag F_(w), is 2, the conveyancecontroller 12 selects and commands one of the carrier vehicles, which isfree and has the maximum mileage, to move to the standby station.

[0049] While the carrier vehicle to be held at its standby state isselected from free carrier vehicles in this example, it might beselected from all the carrier vehicles on the system including thein-action and reserved vehicles in the case of absence of free vehicles.In the latter case, the in-action and reserved carrier vehicles can bedriven to the standby stations whenever finishing the current conveyingaction or the conveying action to be conducted.

[0050] When the number N₂ is smaller than the optimum number N₁ (N₂<N₁),the carrier vehicles in action are too few for handling the load in theconveyance system. Then, some of the carrier vehicles have to bereleased out from the standby stations for conveying actions. Theconveyance controller 12 accesses the carrier vehicle priority table 28to examine the in-action priority type flag F_(D). When the in-actionpriority type flag F_(D) is 1, the conveyance controller 12 selects fromthe carrier vehicle status table 24 one of the carrier vehicles which isat the standby state and has the minimum mileage and commands it tostart its conveying action. When the in-action priority type flag F_(D)is 2, the conveyance controller 12 selects and commands one of thecarrier vehicles, which is at the standby state and has the maximummileage, to start its conveying action. Alternatively, when thein-action priority type flag F_(D) is 3, the conveyance controller 12accesses the conveyance command table 20 to retrieve a command which isnot yet assigned with the carrier vehicle and determine its destinationstation. The conveyance controller 12 then selects from the carriervehicle status table 24 one of the carrier vehicles, which is locatednearest to the destination station, to start its conveying action.

[0051] When the number N₂ is equal to the optimum number N₁ (N₂=N₁), thenumber of the carrier vehicles is proper for conveying the load in thesystem and no more action for controlling the number of the carriervehicles will be needed.

[0052] As described, this embodiment of the present invention allows thenumber of carrier vehicles to be automatically determined according tothe conveyance command number, i.e. the number of cargoes to beconveyed. Accordingly, even if the load to be handled by the conveyancesystem is not uniform but varied, the conveying action can be carriedout promptly and efficiently.

[0053] Also, since each of the stations for loading and unloadingcargoes is accompanied with the standby station for standby actions, thecarrier vehicle which has just finished its unloading action can readilybe held at its standby state.

[0054] Moreover, either the carrier vehicles to be used or the carriervehicles to be held at their standby state can be determined dependingon their mileage. As a result, the timing of maintenance of the carriervehicles can be optimized by modifying their mileage. For example, themaintenance can be performed at the same timing by making the mileageuniform. In reverse, the timing of maintenance might be set differentamong the carrier vehicles for avoiding their maintenance actions frombeing conducted at once.

Embodiment 2

[0055] In Embodiment 1 of the present invention, the conveyance commandnumber is calculated at the end of every conveying action and utilizedto determine the number of the carrier vehicles to be used.

[0056] It is however possible to predetermine the number of the carriervehicles to be used whenever the upper-stage computer 10 releases acommand for the conveying action.

[0057] When the command for the conveying action is released from theupper-stage computer 10, its start (the dispatch station) and goal (thedestination station) are saved in the conveyance command table 20. Theconveyance controller 12 accesses the conveyance command table 20 todetermine the conveyance command number n at the current time. Asdescribed, the conveyance command number n is the number of conveyancecommand recorded in the conveyance command table 20.

[0058] Then, the conveyance controller 12 calculates the optimum numberN₁ of the carrier vehicles from the carrier vehicle number determiningtable 22 and determines the number N₂ of the carrier vehicles in theirin-action state from the carrier vehicle status table 24.

[0059] When the number N₂ is greater than the optimum number N₁ (N₂>N₁),some of the carrier vehicles at their in-action state are driven to thestandby stations. As described, the carrier vehicles to be driven to thestandby stations are selectively determined on the basis of the standbypriority flag F_(w) of the carrier vehicle priority table 28. When thenumber N₂ is smaller than the optimum number N₁ (N₂<N₁), some of thecarrier vehicles at their standby state are released out from thestandby stations. Similarly, the carrier vehicles to be released areselectively determined on the basis of the in-action priority type flagF_(D) of the carrier vehicle priority table 28. When the number N2 isequal to the optimum number N1 (N2=N1), no action for controlling thenumber of the carrier vehicles will be needed.

[0060] As described, this embodiment in which the number of carriervehicles is adjusted when the command for conveyance is given, likeEmbodiment 1 for controlling the number of carrier vehicles at the endof each conveying action, can also determine the number of the carriervehicles to be used on the basis of the conveyance command number, i.e.the number of cargoes to be conveyed.

[0061] Of course, the number of carrier vehicles can controllably bedetermined at both the timing of the end of every conveying action andthe timing of release of another command for conveying. A differencebetween the optimum number N₁ of carrier vehicles and the number N₂ ofcarrier vehicles at their in-action state is caused by a change in theconveyance command number n, i.e. due to the end of the conveying action(as n being decreased) or the release of another command for conveying(as n being increased). Accordingly, if the number of the carriervehicles is controllably determined at both the timing of the end ofevery conveying action and the timing of release of another command forconveying, it is possible to readily correspond to a change in theamount of load to be conveyed, thereby adjusting the number of carriervehicles.

Embodiment 3

[0062] The conveyance controller 12 which receives a command forconveying from the upper-stage computer 10 selects and directs thecarrier vehicle for performing the conveyance to move to the dispatchstation. According to the previous embodiments, the carrier vehicle isselected from the carrier vehicle at their in-action state. Thus, evenif one carrier vehicle at its standby state is available near thedispatch station, it will not be selected. As a result, the carriervehicle selected has to be driven from its current far location to thedispatch station thus creating an idle time and declining the smoothnessof its conveying action.

[0063] This embodiment selects the optimum carrier vehicle from thecarrier vehicles at both the in-action and standby states, i.e. all thecarrier vehicle excluding those under the maintenance condition.

[0064] As described, when a command for conveying is issued from theupper-stage computer 10, its dispatch station and destination stationare firstly saved in the conveyance command table 20. The conveyancecontroller 12 accesses the carrier vehicle status table 24 to select thecarrier vehicle to be used. While the carrier vehicle status table 24has data about the current location of the carrier vehicles determinedfrom signals of the stations and the check points, the conveyancecontroller 12 selects one of which the current location is nearest tothe dispatch station from the carrier vehicles in-action state andstandby state. The carrier vehicle selected moves to the dispatchstation for loading with a cargo and then runs to the destinationstation for delivery.

[0065] The present embodiment allows the carrier vehicles at theirstandby state to be candidates for selection of the carrier vehicle tobe used for the conveying action. Since any carrier vehicle which isnearest to the dispatch station is used, the idle time can be decreasedand the waste running can be minimized.

[0066] When the carrier vehicle at its standby state is selected, thenumber of the carrier vehicles at their in-action state is increased andmight be too large for the load in the conveyance system. This can beovercome by controlling the number of the carrier vehicles at the end ofevery conveying action as described in Embodiment 1.

Embodiment 4

[0067] This embodiment will be described referring to the flowchart ofFIG. 10.

[0068] As described previously, the conveyance system of the presentinvention saves the mileage of each carrier vehicle in the carriervehicle status table 24. When the interval of maintenance actions isrecorded in the carrier vehicle status table 24 and used for comparisonwith a mileage of the carrier vehicle to determine whether themaintenance action is needed or not, each carrier vehicle canautomatically be driven to the maintenance station upon finding that itsmaintenance action is needed.

[0069] The signal indicating the passage of the carrier vehicle istransmitted from each check point to the conveyance controller 12. Uponreceiving the signal, the conveyance controller 12 accesses the CPdistance table 26 to determine the distance d between the current checkpoint and the preceding check point where the carrier vehicle has beentraveled. Then, the distance d is added to the mileage D_(T) of thecarrier vehicle saved in the carrier vehicle status table 24 by theconveyance controller 12.

[0070] The resultant sum of the mileage D_(T) is compared with themaintenance distance D_(M). When its mileage D_(T) is not smaller thanthe maintenance distance D_(M)(D_(T)≧D_(M)), the carrier vehicle iscommanded to move to the maintenance station 5 by the conveyancecontroller 12. When the carrier vehicle arrives the maintenance station5, a signal indicative of the arrival is released from either thecarrier vehicle itself or the maintenance station 5. Upon receiving thearrival signal, the conveyance controller 12 notifies the upper-stagecomputer 10 of the arrival of the carrier vehicle at the maintenancestation 5.

[0071] This embodiment allows any carrier vehicle which has run apredetermined length of distance to be automatically driven to themaintenance station 5 for the maintenance action.

[0072] Although the interval of maintenance actions shown in the carriervehicle status table of FIG. 6 is uniform, it might be set differentbetween the carrier vehicles depending on the type, the age, and thehistory of faults.

Embodiment 5

[0073] According to the previous embodiments, the track has double-tracksections where the stations 3 for loading and unloading of cargoes andthe standby stations 4 for the standby action are provided. However, theconveyance method of the present invention is applicable to amodification where the stations 3 (3 d, 3 e, and 3 f) are directlyprovided across the main line of the track 2 as shown in FIG. 11. Also,the conveyance method of the present invention is applicable to anothermodification where the standby stations 4 (4 g, 4 h, 4 i, and 4 j) areprovided on bypasses of the track 2.

[0074] Moreover, the conveyance method of the present invention isapplicable to a further modification where two or more tracks 2 and 2 aare provided as they share a common section, as shown in FIG. 12. Thenumber of carrier vehicles can be controlled while the tracks aretreated as either different systems or a single system.

[0075] As set forth above, the present invention allows the number ofthe carrier vehicles to be automatically determined corresponding to thequantity of load in the conveyance system. Accordingly, even if the loadis not uniform but significantly varied, the conveying action can beconducted readily and efficiently. Also, since waste running of carriervehicles is minimized, the conveyance system can favorably beimplemented while increasing the operational life and minimizing theenergy consumption.

[0076] The present invention also allows the carrier vehicle to beselected from all the available carrier vehicles including one at theirstandby state and quickly forwarded to the dispatch warehouse (station)thus minimizing the standby time and increasing the speed of theconveying action. Also, since the mileage of each carrier vehicle isminimized, the conveyance system can be improved in both the energysaving and the operational life

[0077] In addition, since the mileage of each carrier vehicle isacknowledge, it can be used for determining the timing of maintenanceand automatically driving the carrier vehicle to the maintenancestation.

[0078] Since the carrier vehicle are classified by mileage into those attheir in-action state and those at their standby state, their mileagelevels can be average for making their maintenance actions be conductedat once. In reverse, particular carrier vehicles can be given priorityfor the maintenance, so that the carrier vehicles are varied in thetiming for maintenance action to avoid their maintenance actions beconducted at once.

What is claimed is:
 1. A conveyance system which performs conveyingaction with operating a part of or all of a plurality of carriervehicles, comprising a loop track, a plurality of carrier vehicles whichrun on the track, a station where each carrier vehicle performs loadingand unloading of a cargo, and a standby station for storing the carriervehicles at their standby state, wherein the number of the carriervehicles to be used is determined from a conveyance command number. 2.The conveyance system of claim 1, wherein the system further includes aconveyance command table for storing conveying action command data aboutthe dispatch stations and the destination stations, a carrier vehiclenumber determining table for storing the optimum number of the carriervehicles with each conveyance command number, and a carrier vehiclestatus table for storing data about the status of the carrier vehicleswhether they are at in-action state or standby state, and wherein theoptimum number of the carrier vehicles is determined from the conveyancecommand number on the conveyance command table and the carrier vehiclesin-action state can be shifted to the standby state or the carriervehicles in the standby state can be shifted to the in-action state sothat the optimum number is equal to the current number of the carriervehicles at their in-action state.
 3. The conveyance system of claim 1,wherein the system further includes a conveyance command table forstoring conveying action command data about the dispatch stations andthe destination stations, and a carrier vehicle status table for storingdata about the current location of each carrier vehicle, and wherein oneof the carrier vehicles which is located nearest to the dispatch stationis assigned with the conveyance command.
 4. A method of controllingconveyance system comprising the steps of: counting the number ofconveying actions either in operation and to be carried out; calculatingan optimum number of the carrier vehicles corresponding to the number ofconveying actions; counting the number of the carrier vehicles at theirin-action state; and when the number of the carrier vehicles at theirin-action state is smaller than the optimum number, using some carriervehicles at their standby state.
 5. A method of controlling conveyancesystem comprising the steps of: counting the number of conveying actionseither in operation and to be carried out; calculating an optimum numberof the carrier vehicles corresponding to the number of conveyingactions; counting the number of the carrier vehicles at their in-actionstate; and when the number of the carrier vehicles at their in-actionstate is greater than the optimum number, shifting some of the carriervehicles from the in-action state to the standby state.
 6. Theconveyance system of claim 3, wherein the system further includespassage detecting means provided on the track for detecting the passageof each carrier vehicle, and wherein the current position of eachcarrier vehicle is identified by a signal received from the passagedetecting means.
 7. The conveyance system of claim 1, wherein the systemfurther includes a maintenance station for carrying out a maintenanceaction of the carrier vehicles and a carrier vehicle status table forstoring data about the mileage of each carrier vehicle, and wherein whenthe mileage exceeds a predetermined interval between two maintenanceactions, the carrier vehicle is commanded to move to the maintenancestation.
 8. The conveyance system of claim 7, wherein the system furtherincludes passage detecting means for detecting the passage of thecarrier vehicle through each check point and a check point distancetable for storing the distance between the passage detecting means, andwherein the distance between the passage detecting means and itspreceding passage detecting means is summed with the mileage of thecarrier vehicle.